Heretofore, thinking in the fire protection art has been directed towards the use of multiple sprinklers for the protection of interior areas more than about one to two hundred square feet so as to limit the maximum average area protected by each sprinkler. As the perceived fire threat rises, the recommended protected area of coverage for each sprinkler is reduced. Such thinking has been embodied, for example, in accepted industry standards such as the Standard for the Installation of Sprinkler Systems, NFPA-13, issued by the National Fire Protection Association, and Standard 199 issued by Underwriters Laboratories, Inc. Existing versions of both standards and all prior, superseded versions of both standards are incorporated by reference herein.
NFPA-13 defines various requirements for sprinkler systems utilized in occupied commercial interior spaces or "occupancies" with different fire hazard potentials. The NFPA-13 recognizes three general hazard categories for sprinkler systems: light, ordinary and extra. As defined by the NFPA-13, light hazard occupancies are those where the quantity and/or combustibility of contents is low and fires with relatively low rates of heat release are expected. Ordinary hazard covers those occupancies where the quantity and/or combustibility of the contents is equal to or greater than that of light hazard, ranging from low to high, where the quantities of combustibles is moderate and stock piles do not exceed twelve feet, such that fires with moderate to high rates of heat release are expected. Extra hazard occupancies are those where quantity and combustibility of the contents is very high and flammable or combustible liquids, dust, lint or other materials are present, such that the probability of rapidly developing fires with high rates of heat release is very high. The present invention is directed specifically to the protection of light and ordinary hazard occupancies, which define the bulk of most potential commercial installations.
The aforesaid application Ser. Nos. 07/875,928 and 07/769,917 disclose both upright and pendant frame-type ceiling sprinklers which have been tested and listed by Underwriters Laboratories, Inc. as providing both light and ordinary hazard protection for extended coverage of up to 400 square feet (20.times.20). All of the sprinklers disclosed in these applications are frame-type sprinklers and must be at least partially if not totally exposed beneath the ceiling over the floor area to be protected by the sprinkler.
While such sprinklers are eminently functional, their exposure and appearance generally make them less desirable for applications in which aesthetics may be a consideration in the sprinkler purchase. Accordingly, it would be desirable to provide at least partially recessed and preferably concealed ceiling sprinklers capable of providing extended coverage for at least light hazard applications.